Causes of nest failure of Slavonian grebes

Slavonian grebe breeding population (pairs) in Scotland, 1971-2002.

Slavonian grebes nest on a small number of lakes in northern Scotland, and annual censuses have taken place since 1971. Between 1978 and 1993, the population fluctuated between 60 and 80 pairs, but then declined to only 31 pairs in 2000, the lowest count since annual monitoring began. The number of occupied lakes also halved over the same period, to 15 in 2000.

Previous work has shown that breeding productivity (clutch and chick survival) of Scottish birds was low compared with other European populations (in Scandinavia and Iceland) and that predation of full-grown birds was taking place. The causes of nest failure were often not determined during this study, however, and the predator of adult grebes was unknown. 

Therefore, in 2001 and 2002, miniature 24-hour time-lapse video cameras were placed next to 23 nests with the aim of establishing the frequency with which nests were predated and to identify the predators responsible.

Six nests were predated, and abandoned eggs were predated from two further nests. The otter was the most frequently filmed predator, taking two clutches and an adult grebe plus chicks from the nest. Other predators filmed were stoat, carrion crow, common gull and coot.

Overall, nest predation rates were low (17% probability of nest predation before chicks hatched), and were similar to failure rates due to flooding (25%), and desertion or accidental or deliberate removal of eggs by incubating grebes (17%). Wave damage was a greater cause of nest flooding than water level rise, and was the most frequent cause of nest failure at exposed sites on Loch Ruthven. 

One constraint of nest cameras is that they can only detect predation at the nest. The full extent of predation of grebes remains unknown, and otters, American mink (recorded at one lake during the study), and large predatory fish such as pike, which have been introduced to many lakes in the region, are all potential predators of grebes away from the nest.

Hancock M, Summers R and Butcher N (2002) Predation of Slavonian Grebe nests by Otters. British Birds 95: 390-391. 

This study was supported by SNH and the Jennie S Gordon Memorial Foundation.

Bird guide

• Slavonian grebe